There are many situations in which light rays can be used for determining a physical characteristic of a material. For example, it is sometimes desirable to measure the thickness of a layer that is deposited on top of a substrate. That is, when a layer on top of a substrate is being planarized or otherwise partially removed in a polishing process, one may want to determine (directly or indirectly) the remaining thickness so that too much material is not removed. As another example, when a layer is being deposited on a substrate, one may want to determine (directly or indirectly) the deposited thickness so that too much or too little of the layer material is not deposited. Thus, the purpose of determining the thickness in some situations may be to determine a desired end point of a manufacturing process. In other examples, a physical characteristic such as thickness may be determined for quality control, classification, calibration, compatibility testing, or other purposes.
Chemical mechanical polishing (CMP) is one example of a manufacturing process in which end point determination is performed. For example, CMP is sometimes performed on a wafer or other substrate that includes integrated circuits. An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or isolative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. The deposited layers must be planarized and then polished down to a specified thickness.
CMP is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by many factors, including the pad and slurry combination, the carrier head configuration, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In order to determine the effectiveness of different polishing tools and processes, a so-called “blank” wafer, i.e., a wafer with multiple layers but no pattern, may be polished in a tool/process qualification step. After polishing, the remaining layer thickness may be measured at several points on the substrate surface. The variation in layer thickness provide a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. One approach to determining the substrate layer thickness and polishing uniformity is to remove the substrate from the polishing apparatus and examine it. For example, the substrate may be transferred to a metrology station where the thickness of the substrate layer is measured, e.g., with an ellipsometer. Unfortunately, this process can be time-consuming and thus costly, and the metrology equipment is costly.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Many different factors can cause variations in the material removal rate, including variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate. These variations in turn cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with an ellipsometer. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
Several methods have been developed for in-situ polishing endpoint detection. Some of these methods may involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an isolative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed. Other endpoint detection methods involve impinging a light beam on the substrate and analyzing the light that reflects off the surface. Such analyses may involve monitoring interference fringes in the reflected light or registering how reflectance varies with an angle of incident polarized light.
However, reflectance from a patterned substrate, such as a wafer with integrated circuitry, may depend on a density of the substrate pattern in the area where the light beam impinges. When a light beam impinges on a patterned substrate, light reflected from a high pattern-density area may have a different reflectivity than light reflected from a low pattern-density area. Measurements based on reflectivity therefore may contain noise introduced by the variance in pattern density across the patterned substrate. Accordingly, it may be difficult to compare measurements taken at different locations on the patterned substrate. Moreover, when doing in situ measurements, the accurate location on the wafer may not be known, which further complicates the measurement.